1. Technical Field
The present invention relates to enhancing the accuracy of network delay measurements. More specifically, the present invention provides a method, computer program product, and data processing system for correcting errors in end-to-end network delay measurements caused by clock skew between the two communicating data processing systems.
2. Description of Related Art
The accurate measurement of end-to-end delay between two data processing systems communicating through a network is very important to determining the state of the network. Having accurate measurements of the delay associated with a particular data path within the network aids network engineers as well as network applications in making decisions regarding network routing, network fault detection, and other issues involved in maintaining or using a network.
End-to-end network delay measurements are generally made by having a first data processing system timestamp and send a packet of data to a second data processing system. The second data processing system reads the current time and compares the current time to the timestamp to determine the end-to-end delay (i.e., how long it took the packet to make it across the network). If the two data processing systems' clocks are not perfectly synchronized, however, this measured delay will not be entirely accurate. Moreover, if one clock operates at a faster or slower speed than the other one, the difference between the measured delay and the actual delay will vary over time. This phenomenon is known as clock skew.
It is thus desirable to reduce, if not eliminate, the effect of clock skew on network measurements. A number of authors have proposed methods of reducing the effect of clock skew by making estimates of the clock skew based on empirical measurements, then compensating the measured delay values to correct for the clock skew. From empirical measurements, one can see that the effect of clock skew is an increase or decrease in measured clock delay that changes according to a linear pattern.
Moon, Skelly, and Towsley, in Estimation and Removal of Clock Skew from Network Delay Measurements, Proceedings of the IEEE INFOCOM Conference on Computer Communications, pp. 227-234, March 1999, used a linear-regression approach to develop a linear function to estimate the clock skew. Paxson, in On Calibrating Measurements of Packet Transit Times, Proceedings of the ACM SIGMETRICS, Madison, Wis., June 1998, used a median line-fitting technique to achieve the same goal.
These methods, however, ignore the fundamental notion that delay measurements are bounded by a linear clock skew function. They also degrade in accuracy when network measurements vary considerably or do not follow a normal distribution. As networks are seldom that well behaved, a need exists for a more accurate method of estimating clock skew, so that appropriate correction can be made to the measurements.